A tree is used to symbolize phylogeny because the formation of branches in both of them hold a lot of resemblances. A stem grows in only one direction at the begining, just like an ancient species begins just as one in a population. Then an event happens (a node) which originates 2 different paths for the population, towards one trait or the other (so a branch is created), and now we have 2 species form the original one. And as every node creates a different branch, every event distincts individuals so much that a new species is created. The heigh of our tree would represent the time of evolution, so the lower nodes occurred first that the higher ones. And in the end we obtain a tree of interconnected branches that represent each change that suffered a species to get to the present (the top of the tree).
